high temperature oxidation

The purpose of this research is to investigate the performance of vertical cracks along the thickness of YSZ-40%wtAl2O3 coating and to improve the resistance to oxidation and thermal shock of thermal barrier coatings by creating a layer of YSZ-Al2O3 applied on the YSZ by atmospheric plasma spraying. In this research, YSZ-40%wtAl2O3 powder was synthesized using co-precipitation process and then applied on the YSZ by atmospheric plasma spraying process. High temperature oxidation test at 1100 ˚C and thermal shock test at 1000 ˚C were performed. The structural and phase characteristics of the coatings were investigated using optical microscope, field emission scanning electron microscope (FE-SEM) and X-ray diffractometry (XRD). The structural comparison of the samples showed that the use of the YSZ-40%wtAl2O3 eutectic compound due to the decrease in the melting temperature of the compound and the increase in the melting which brings the proper contact between the splats. This coating showed 18.6% increase in oxidation resistance compared to conventional TBC. Also, the findings showed that the decomposition of the un-pyrolysed precursor in the coating structure of YSZ-40%wtAl2O3 leads to the creation of vertical cracks in the coating structure, which as the oxidation time increases, the number of vertical cracks with a certain distance in the coating structure will also increase. Also, the suitability of the vertical cracks created in the YSZ-40%wtAl2O3 coating, in terms of stress tolerance, increases the ability of the coating to release the stresses during thermal cycles and finally leads to increase in the durability of the coating.

In this research, CoNiCrAlY powder was deposited by high-velocity oxy-fuel (HVOF) and low-pressure plasma spraying (LPPS) processes on IN738 nickel-based superalloy substrates. The high-temperature oxidation test was performed on the coatings at a temperature of 1050 ̊C and a time of 200 hours in a muffle furnace. The microstructure and phase composition of the coatings were investigated by SEM and XRD before and after the oxidation test. The porosity (volume percentage) and surface roughness (micrometer) were measured for HVOF coating as 0.6 and 4.4, and for LPPS coating as 2 and 6.62, respectively. The HVOF coating consisted of γ-CoNiCr and β-(Co,Ni)Al, while the LPPS coating included a single phase γ-CoNiCr. The disappearance of the β phase in the LPPS coating after spraying was due to dissolution in the plasma jet and its non-recovery in the conditions of rapid quenching and non-equilibrium solidification. This phase was recovered after heat treatment. The microstructure of the LPPS coating had much less oxide than the HVOF coating due to depositing at low oxygen pressure in the vacuum chamber. After 200 hours of oxidation test, the amount of β phase (as an oxidation resistance criterion) was completely consumed in the LPPS coating, while the HVOF coating contained the retained β deposits. The average thickness of TGO layer for HVOF and LPPS coatings was 5.2 and 7.1 μm, respectively. The dispersed oxides in the microstructure, lower roughness and denser structure of HVOF coating were reasons for the higher oxidation resistance of HVOF coating than LPPS.

In this study the effect of UTP 2535 Nb and UTP 3545 Nb filler metals on the microstructure and oxidation resistance of the weld metal of GTAW welded 35Cr-45Ni alloy have been investigated. According to the results, microstructure of the weld metals includes Cr and Nb Carides precipitating along the grain boundaries of austenitic matrix. Weld metal of the sample welded with UTP 3545 Nb filler metal showed denser and more continuous carbides which arises from higher Cr content in this filler metal. According to the XRD analysis of the oxidized weld metals at 1000 ℃ for 96 h, all samples represent an external oxide layer, mostly consist of Cr oxide and an internal oxide layer, mostly composed of Si oxide. Weld metal of the sample welded with UTP 2535 filler metal showed the highest weight gain (4.3 mg/cm2) and oxide thickness (20 μm). For the weld metal of the other sample, these values were 3.6 mg/cm2 and 11 μm, respectively. It can be attributed to the higher Cr and Si in the UTP 3545 Nb filler metal. The first element results in a resistant, continuous, and defect free external oxide on the surface, and the latter one leads to formation of a resistant internal oxide, both strongly effective in improving the oxidation resistance of the weld zone.

The present study investigates the oxidation behavior of GTD-111 nickel base superalloy at 1000 °C in air. To this aim, several techniques including thermal gravimetric analysis (TGA), field emission scanning electron microscopy (FE-SEM), grazing incidence X-ray diffraction (GI-XRD) and glow discharge optical emission spectroscopy (GDOES) were employed. The oxidation kinetics of this superalloy alters from linear to parabolic after 10 h exposure which is accompanied by the formation of a uniform surface oxide layer. Based on the cross-sectional analysis of FE-SEM and GDOES, Ti can most probably be considered as the first species which enters the reaction front when oxidation begins. However, as a result of its oxidation, Ti is depleted just beneath the oxide scale, and conditions changes in favor of the formation of other oxides, namely, Cr2O3. Subsequently, due to the internal diffusion of O2- species, Al is internally oxidized, and islands of Al2O3 form underneath the oxide layer. After 100 h of oxidation, the chemical composition of the oxide film consists of an Al-rich inner region, a Ti-rich outer region, and a Cr-rich intermediate layer. GI-XRD results also confirmed the formation of mentioned oxide phases. No indication of oxide spallation and formation of nickel oxides were observed even after 100 h of oxidation.

The high-temperature erosion-oxidation behavior of a Ni-20Cr-3Mo-4Fe alloy containing 0, 2, and 4 wt.% of Si was studied. The specimens were exposed to 700 °C silica sand mixed with 0.5 wt.% of NaCl-KCl salt for 250 h. The difference in the thickness of the samples before and after the erosion-oxidation test was the measure for the material loss. The sample comprising 4 wt.% of Si showed a thickness reduction of about 45 percent higher than the Si-free specimen. Since the oxide scale is mostly removed by the eroding particles, the specimens were oxidized in the atmospheres of moving air and air+chlorine in a tube furnace at 520 °C for 100h to better understand the behavior of oxide scales. The Si bearing alloys were interestingly of higher corrosion resistance. The results of microscopic evaluations were indicative of the effects of Si on the reduction of oxidation rate, deteriorating spalling resistance, and increasing the Cr interdiffusion coefficient in the alloy and a subsequent reduction in the Cr-depleted zone.

The bare and pre-oxidized AISI 430 pieces were screen printed by copper ferrite spinel coatings. Good bonding between the coating and the substrate was achieved by the reactive sintering process of the reduced coating. The energy dispersive X-ray spectroscopy (EDS) analysis revealed that the scale is a double layer consisting of a chromia-rich subscale and an outer Cu/Fe-rich spinel. The results showed that the spinel protection layer not only significantly decreased the area specific resistance (ASR), but also inhibited the subscale growth by acting as a barrier to the inward diffusion of oxygen. ASRs of 19.7 and 32.5 mΩ.cm2, much lower than that of the bare substrate (153.4 mΩ.cm2), at 800 °C after 400 h oxidation were achieved for the bare and pre-oxidized copper ferrite spinel coated samples, respectively. Excellent, stable ASR (20.5 mΩ.cm2) was obtained with copper ferrite coating after 600 h of exposure at 800 °C. The high electrical conductivity of CuFe2O4 and its doping by Mn, the growth reduction of Cr2O3 oxide scale and the good coating to substrate adherence are proposed to be responsible for substantial improvement in electrical conductivity.

In this research, a double layer thermal barrier coating was applied and then an alumina diffusion barrier layer was deposited on the YSZ by two solution precersore plasma and solution precersore flame spraying. High temperature oxidation and thermal shock resistance tests were done at 1100˚C. Microstructure of coatings were studied by optical Microscopy and Field Emission Scanning Electron Microscopy. Comparison of the microstructures of coatings showed that applying of Alumina with the solution precursor flame spray process upgrades the thermal properties. High temperature oxidation and thermal shock resistance of YSZ/Al2O3 coatings with Alumina applied by the solution precersoure thermal spray with the same compound were studied. Findings showed that applying alumina with the solution precursor flame spray process leads to increase the amount of the deposited splats and proper contact between them, causes to decrease the diffusion of O2 and as a result TGO thickness decreases and also thermal shock resistance increases.

Thermal barrier coatings (TBCs) as one of the high temperature coatings are widely used to improve the performance of the blades of turbine engines and in particular gas turbines. The purpose of this study was to investigate the thermo-mechanical properties, hot corrosion resistance and high temperature oxidation of conventional zirconia TBC coatings and improve their performance by adding cerium oxide stabilizer to the compound. For this purpose, CoNiCrAlY/YSZ and CoNiCrAlY/CYSZ coatings were applied to substrates of IN738LC by atmospheric plasma spraying. Thermomechanical behavior of the coatings was investigated by measuring the thermal expansion of the coatings. Hot corrosion behavior and oxidation of the coatings by furnace method were investigated at 1050 °C and 1100 °C with 4-hour cycles, with and without the applied of a mixture of corrosive salts (Na2SO4 -V2O5 ). The results showed that CYSZ coating has a higher thermal expansion coefficient than YSZ coating. Microstructural and phase studies of coatings by SEM/EDS and XRD analysis before and after hot corrosion and oxidation tests showed better resistance to CYSZ coating than YSZ coatings.

The aim of this work, the behavior of hot corrosion and thermal stability of nanostructured thermal barrier coating made by yttria stabilized zirconia (8YSZ) was studied. For this purpose, on the stainless steel substrates double layer of NiCrAlY with thickness of 150 μm and nanostructured YSZ with thickness of 300 μm was deposited using atmospheric plasma spraying process. The high temperature cyclic oxidation test at 1000oC and 10-hour periods was performed in order to check the coating resistance against oxidation and thermal cycles. Type I hot corrosion test was carried out at 950oC for 4-hour combining with salts such as V2O5 and Na2SO4. The results show that the nanostructured coating with grain sizes less than 100nm in order to high oxygen ion permeability did not have a significant effect on reducing the TGO layer growth. However, the results of the hot corrosion test were showing that the formation of YVO4 crystals on the surface of the nanostructured YSZ coating. Due to the high diffusionability of nanostructured coating from the top surface to the depth of YSZ some vanadium diffused to the lower layer of YSZ nanostructured coating.

In this study, microstructure investigation, micro-hardness, fracture toughness and oxidation behavior of stellite6 and NiCrBSi coating produced with HVOF and GTAW methods has been investigate. The microstructure investigation of raw materials and coatings do with Secondary Electron Microscopy (SEM) and Optical Microscopy (OM). A Vickers micro-hardness meter used to study of hardness of coatings with 300 gr load. The fracture toughness of coatings investigated with Vickers hardness meter on the 1.5 to 200 Kg load. Oxidation behavior investigation of coatings carried out at 750 °C for 2, 5, 15 and 50 hrs. The oxide layer formed on the coatings during oxidation test studied with X-Ray Diffraction method. The oxidation results show the layer of Cr2O3 formed on the thermal sprayed Stellite6 coating. It caused to this coating show lower oxidation rate among all coatings. But, on the oxide layer of Stellite6 cladding coating, the spinel oxide phases are major phase, that are formed due to complex phase formed during surface welding.

In this research, effects of different coating process parameters on the final properties of thermal spray coatings obtained from HVOF method are investigated. For this aim, five different levels are considered for four effective parameters (Fuel injection rate, oxygen injection rate, powder injection rate and spray distance) on HVOF process by response surface method (RSM) and minimum porosity and minimum oxidation layer thickness are considered as the responses. To investigate the microstructure of coatings, Thermal Grown Oxide (TGO) thickness and porosity, a Scanning Electron Microscopy (SEM) and image analysis method are applied. Oxidation test has done at1100ºC for 50 hours on coated samples. Results show that (oxygen injection rate and powder injection rate), (fuel injection rate and powder injection rate), (powder injection rate and spray distance) have a reaction on each other in the TGO thickness value. Also, (oxygen injection rate and powderinjectionrate), (fuel injection rate and powder injection rate) have a little reaction on the porosity against the other parameters which haven’t any effect on it. Investigation of the coated sample with optimized parameters reveals that porosity of the coating is about 0.5% and TGO thickness is about 5.3 micrometer.

The isothermal high temperature corrosion behavior of an Fe3Al coating coated on plain carbon steel through tungsten inert gas (TIG) surface alloying with current of 40A was studied in the atmosphere of air. The specimens were tested at 700 and 900 °C for 4–100 h. SEM, EDS and XRD analyses were utilized to study the oxidation behavior of the iron aluminide intermetallic coating. The weight gain data determined a parabolic rate law. The results also revealed that the initial scale formed on the coating was predominately θ-Al2O3 which was substituted by α-Al2O3 after about 64 hours. The SEM observation and XRD results illustrated that at shorted times, when the θ phase was stable, spalling was more usual while with the increase in the amount of the α phase the spalling decreased significantly. The EDS results confirmed the presence of aluminum oxide at shorted times and iron oxide at longer times.

Surface alloying of aluminum into AISI 316 austenitic stainless steel was prepared by electroplating nickel layer followed by a single-step high-activity aluminizing process. After aluminization, the surface microstructure of stainless steel consist of β-(Fe, Ni)Al as matrix with β-FeAl+ α-(Fe, Cr) precipitates. Aluminized coating on specimens with 10 and 20 μm primary thickness of nickel layer made up of two layers; a thin layer of β-NiAl and a β-(Fe, Ni)Al layer. In the case of the nickel layer with thickness of 50 μm, the coating showed three layers: β-NiAl and Ni3Al phase and layer of remained Ni was also determined. Oxidation test at 950ºC was carried out for 157 hours to evaluate the coatings. By increasing primary electroplated nickel to 100μm, the weight gain of aluminized specimens decreased from 2.95 to 1.44g/cm2. Surface morphology of oxide scale on Th 100 after oxidation for 160 h at 950°C showed the stable α-Al2O3 and it can be confirmed from the XRD results.

In this study the effect of addition YSZ on structural properties and oxidation resistance of CoNiCrAlY/YSZ composite were investigated. CoNiCrAlY/YSZ composite powders produced by mechanical alloying process. Various amounts of YSZ particles (0%, 5%, 10% and 15 wt.%) were mixed with CoNiCrAlY powder and milled for 12, 24 and 36 h, then composite and commercial coating were deposited with high velocity oxy -fuel method on the inconel 617 substrate. The structural and mechanical evolutions of the coatings were evaluted using X-ray diffractometry, scanning electron microscopy and microhardness test. It was observed that by increasing YSZ content, porosity of coatings were increased and hardness of coatings were increased too. Also, addition of YSZ to CoNiCrAlY decreased the rate of grain growth in thermal spray process and heat treatment. Increase of porosity result to decreasing oxidation resistance of coatings.

In this study, Mo-14Si-10B and Mo-57Si-10B (at%) elemental powders were separately milled using an attritor mill.Mechanically alloyed powders were agglomerated and annealed. Then, powders of Mo-Si-B as alloyed (with composites) and agglomerated (without composites) were plasma sprayed onto plain carbon steels. The samples, both coated and noncoated,were subjected to isothermal oxidation tests. Metallurgical characteristics of powders and coatings were evaluated by SEM and XRD. Plasma-sprayed Mo-Si-B coatings (with phases of MoSi2, Mo5Si3, MoB and Mo5SiB2) greatly improved the oxidation resistance of the plain steel substrates, but plasma-sprayed Mo-Si-B coatings (without any phases) did notsignificantly improve the oxidation rate of substrates. Also, the kinetics and composition of the oxide-scale have been found to depend on the alloy composition.